The invention relates to a digital signal transmission system which operates by coded modulation of a constellation, said system comprising an encoder which includes a modulator installed at a transmitting station, and a decoder which includes a demodulator installed at a receiving station, the encoder comprising a multistage channel encoder which combines a channel coding with the coded modulation, the channel encoder comprising an assigning element for assigning the signals of the stages to points of the constellation and for supplying symbols.
The invention also relates to an encoder and a decoder used in such a system.
The transmission system is mainly used for transmitting digital television signals (audio and/or video) to portable receivers, mobile receivers or others.
This may relate to transmitting digital television signals over a satellite channel or digital signals by way of hertzian waves. This may likewise relate to transmitting sound by mobile radio, or digital data to be stored, for example, on a compact disc, a digital video recorder. In these cases it is necessary first to effect a rate reduction at the source by a source encoder at the transmitting end and to restore the rate by a source decoder at the receiving end. In the cases where one wishes to transmit digital data between two digital processing units, for example, between two computers, this rate reduction by a source encoder is not necessary.
The most representative use is that relating to digital television for which the problems are the most extensive and the present patent application is filed for this use without this constituting any restriction whatsoever.
The digitization of television signals produces an overall gross rate for which a transmission cannot economically be realized over the existing transmission channels. Various coding techniques have been developed to reduce the rate (source coding). The performance of these coding algorithms is measured in terms of their "rate reduction factor" and of the picture quality recovered after decoding. The more the redundancy of the signal is reduced, the more the transmitted information is significant. Possible transmission errors, which may very easily be corrected if the transmitted information signals are redundant, have all the more serious consequences when the reduction factor increases.
Consequently, the transmission of digital television signals calls for a suitable protection. In order not to perceive the effects of transmission errors on a screen, the line error rate is to be less than 10.sup.-10.
The channel used for broadcasting by ground-based transmission is characterized by:
an overall bandwidth of 8 MHz, the effective bandwidth being of the order of 7 MHz,
the presence of noise, considered additive white Gaussian noise,
selective fading due to the multipath propagation of the transmitted signal.
The known source coding techniques, for example, based on an orthogonal transformation, can be utilized for diminishing the rate by a factor of more than 10 while a good recovered picture quality is ensured. This leads to transmission at a binary rate of the order of 8 Mbits/s. The transmission of digital television signals over such a channel makes a source coding as well as a digital modulation necessary with a spectral efficiency of the order of 1-2 bits/s/Hz.
Then it is necessary to perform a channel coding to protect the transmission against channel imperfections. The coding techniques and conventional modulation techniques have turned out to be restricted as regards fully satisfying the requirements of correct transmission (in these techniques, the coding function is considered an independent entity from the modulation function). However, the coding techniques have considerably improved thanks to the coding technique proposed by G. UNGERBOECK in the article entitled: "Channel Coding with Multilevel/Phase Signals", published in IEEE Transactions on Information Theory, Vol. IT-28, No. 1, January 1982, pages 55-67.
There is proposed to regard the channel coding and the modulation as one entity and, therefore, to combine a channel coding with a digital modulation. This makes it possible to increase the efficiency of digital transmission, thus to improve the performance without sacrificing the spectral efficiency. The redundancy added by the coding is transmitted by the increased size of the alphabet in lieu of diminishing the data rate. This technique is based on the principle of maximizing the minimum Euclidean distance between the transmitted coded point sequences.
Thus, after a coding which transforms p information bits into m bits, with m&gt;p, where m-p represents the added redundancy for the protection of information signals during transmission, with a 2.sup.m state modulation, there are 2.sup.m-p additional states available to transmit this redundancy. This modulation technique makes it possible to spread the redundancy over space and not over time.
At the end of the discovery by G. UNGERBOECK of the trellis coded modulations (TCM), block coded modulations (BCM) and trellis coded multi-dimensional modulations have been proposed.
Moderately complex TCMs (4 or 8 states) can provide a coding gain of 3 to 4 dB. But, in the mass production applications, an installed Viterbi decoder necessary for decoding these TCMs remains costly with current technology. An attractive coding technique for these applications is that of the multilevel coding. The importance of this technique is that it is adapted to a simple sub-optimum decoding method which is performed in stages and presents a good compromise between performance and complexity of installation.
Based on the partitioning principle of a constellation revealed by G. UNGERBOECK, the use of multilevel coding has particularly been analyzed by G. J. POTTIE and D. P. TAYLOR in "Multilevel Codes Based on Partitioning", IEEE Transactions on Information Theory, Vol. 35, No. 1, January 1989, pages 87-98. They describe a digital signal transmission system of the type defined in the opening paragraph of this description.
In their article these authors first analyze the principle of the multilevel coding which consists of partitioning a constellation and coding the points of the constellation and, secondly, define a multistage encoder, one coding stage being associated to one partitioning level, the points being transmitted in blocks through a transmission channel. This article considers the case of a single constellation.
At the receiving end a multistage decoder performs the reverse operations to those effected during the coding and recovers points that correspond to the transmitted points. In a conventional decoder, this causes decision operations to be performed during which operations points are estimated and bits are determined for the codes of the estimated points as a function of the phase and amplitude which are detected at the receiving end for each received point. Depending on the different conditions of transmission and reception, certain estimated bits are erroneous. A first multilevel decoder stage decides thereafter the first partitioning level. The result produced by this first stage is utilized for enabling the operation of the second stage and so forth up to the last stage.
For coding the redundancy, these authors use a channel encoder formed by an internal encoder followed by an external encoder. The internal encoder performs a convolutional coding which is concatenated with a parity coding performed by the external encoder. The codewords are then converted into coded points of a constellation and then transmitted by a modulator. The use of block codes is also mentioned.
Such a technique, however, is insufficient when the channel frequency response is subject to fading. It is also insufficient when the channel changes in the course of time. This is particularly the case when a mobile receiver is concerned, which may be subjected to very much varying and often very disturbed receiving conditions. It is thus not possible under these conditions to ensure a signal-to-noise ratio which is sufficient to guarantee correct reception without introducing a large complexity of hardware means.
It is thus an object of the invention, for a power level determined at the transmitting end, to ensure a correct reception while a small signal-to-noise ratio is tolerated. This is to be achieved while minimizing the costs and complexity of necessary hardware.